1. Field of the Invention
The present invention relates to a porous film manufacturing apparatus and, more particularly, to an apparatus for manufacturing porous films suitable for, e.g., an easily tearable film and a gas permeability-controlled film.
2. Description of the Related Art
French Patent 2073807 discloses a thermoplastic film fine perforation method including the steps of providing a pair of rolls which have a pinching portion between them and between which a fixed distance is held, applying abrasive particles on the surface of one of the rolls, passing a thermoplastic film through the pinching portion of the rolls, perforating the film with the abrasive particles by adjusting the pressure and the temperature of the pinching portion of the rolls, finely perforating the film with corner portions of the abrasive particles in contact with the film, and removing the finely perforated film from the pinching portion of the rolls. The abrasive particles on the surface of one of the pair of rolls are applied by bonding sandpaper, such as garnet type sandpaper, on the circumferential surface of the perforating roll. In this apparatus, however, if the pair of rolls are brought into contact with each other and a thermoplastic film is passed between the rolls, the garnet type sandpaper adhered as abrasive particles on the surface of one of the rolls is damaged by perforation in a short time period, since the strength of the sandpaper itself is low. As a result, it is difficult to continuously form through pores in thermoplastic films over long periods of time.
German Patent No. 2,830,402 discloses a method by which a large number of through pores are formed in a polyethylene or polypropylene film by passing the film through a pinching portion between a hard roll, which is covered with hard particles such as corundum particles or SiC particles, and a counter roll. According to this German Patent, the resultant film is applied to tea bags.
The applicant of the present invention, on the other hand, has previously filed a porous film manufacturing apparatus comprising: feed means for feeding an elongated film; a perforating unit including a first rotatable roll having a surface on which a large number of particles, each having acute corner portions and a Mohs hardness value of 5 or more, are deposited, and a second roll which is rotatable in a direction reverse to a rotating direction of the first roll, the first and second rolls being arranged to oppose each other and to cause the elongated film to pass therebetween, one of the rolls being stationary and the other roll being movable in a direction to oppose the one roll; and pressure control means, arranged near two end portions of the movable roll of the unit, for controlling a pressure applied from each of the rolls to the film. This manufacturing apparatus was filed in Japan and in various foreign countries including the United States and European countries, and is allowed as U.S. Pat. No. 5,257,923 (Nov. 2, 1993) in the United States and disclosed as European Patent Application No. 0,502,237 A1 by EPC.
This manufacturing apparatus can manufacture porous films in which through or non-through pores with a small size arbitrarily selected within the range of sub-.mu.m to ten-odd .mu.m are uniformly formed at a high density in elongated films made of various materials including polymeric materials and metals, with almost no deterioration in the essential characteristics of the film materials.